This invention relates to a method for compression and transmission of video information, and is particularly useful in, but not limited to, facsimile transmission systems employing digital data transmission.
The transmission of digital video information such as images of pictures, graphs, and text material usually employs a form of pulse code modulation (PCM) and requires the transmission of a large number of picture elements ("pixels" or "pels").
In digital facsimile transmission, an original image such as a picture or other document is scanned by a photosensor in a series of vertically spaced parallel horizontal lines, i.e. in a manner generally similar to (but usually much slower than) the scanning of a television image. The output of the photosensor is sampled in synchronism with the scanning of the original. Each sample, corresponding to the darkness of one "dot" or pixel of video information, is then converted from analog form to a digital byte consisting of a number of bits, the value of which byte corresponds to the amplitude of the video signal generated by the photosensor.
The transmission path, whether a telephone line or a wireless channel, is usually of limited bandwidth, so that in many cases the transmission time is longer than is desirable.
To reduce the transmission time of such video information, signal compression techniques are commonly employed. These techniques reduce the amount of data which must be transmitted by reducing redundancy in the video information and/or sacrificing gray scale range or resolution.
The present invention deals with transmission of digital video information, and applies to all systems in which the video information is digital in nature, even though it may be transmitted as a digitally modulated analog signal or in some other form.
Such facsimile and other video information transmission systems are well known in the art. See, e.g., U.S. Pat. No. 4,547,810 entitled Portable Facsimile Transmitter (the disclosure of which patent is incorporated herein by reference). Also see U.S. Pat. No. 3,678,388; International Application No. PCT/US79/00729 filed in the United States as Priority Application No. 943,523 on Sept. 18, 1978; U.S. Pat. No. 4,153,916; U.S. Pat. No. 3,902,008; U.S. Pat. No. 3,688,039; U.S. Pat. No. 3,673,322; and French Pat. Nos. 72.05254 (Pub. No. 2,125,532), 77.38332 (Pub. No. 2,374,796), and 78.19012 (Pub. No. 2,396,478).
French Pat. No. 72.05254 relates to an analog signal compression technique.
In U.S. Pat. No. 3,688,039 a synchronizing code, which denotes the beginning of each scanned line, is followed by an error detecting code. When an error is detected in the error detecting code at the receiving end, the corresponding video line is replaced with video information from the first preceding line in which an error was not detected.
U.S. Pat. No. 3,902,008 discloses one form of a type of digital data compression known as run length coding, where instead of transmitting sequences of identical bytes, the first byte of each sequence is transmitted together with a code designating the number of identical bytes in the sequence which follows.
It is also known to use run length coding for groups of run length encoded sequences (also known as double run length coding), so that where a group of run length encoded bytes is repeated a number of times, only the first group is transmitted, along with a run length code designating the number of repetitions of that group.
An object of the present invention is to provide an improved method for the compression and transmission of video information.